Conventionally, a direct-type tire pressure detecting apparatus has been known as an example of a tire pressure detecting apparatus. In the direct-type tire pressure detecting apparatus, a transmitter having a sensing unit such as a pressure sensor is directly fixed to each wheel having a tire, and an antenna and a receiver are fixed to a vehicle body. The receiver receives a detection result of the sensing unit from the transmitter through the antenna, and detects an air pressure of the tire based on the detection result.
In such a direct-type tire pressure detecting apparatus, data transmitted from each transmitter is provided with unique identification (ID) information for identifying that the data is associated with a subject vehicle and for identifying a position of the wheel to which the transmitter is fixed. In the receiver, each ID information and each wheel are correspondingly registered. When receiving a frame including the ID information from the transmitter, the receiver determines the position of the transmitter associated with the wheel, that is, which transmitter the frame is transmitted from based on the registered ID information.
For example, it has been proposed to provide a transmitter with a function as a receiver to perform a two-way communication with the receiver so as to identify a position of a wheel, that is, to identify which wheel the transmitter transmitting the data is fixed to. JP2007-015491A, which corresponds to US2007/0008097A1 and is hereinafter referred to as the patent document 1, describes an example of a wheel position identifying device.
In the wheel position identifying device of the patent document 1, a triggering unit that transmits a trigger signal is mounted on a body of a vehicle at different distances from transmitters. Each of the transmitters measures an intensity of the trigger signal from the triggering unit. Using a principle where the intensity of a radio wave attenuates in accordance with a distance, the position of each wheel is identified based on data indicating the intensity of the trigger signal received in the corresponding transmitter.
JP2006-298182A, which corresponds to US2006/0238323A1 and is hereinafter referred to as the patent document 2, describes another example of a wheel position identifying device. In the described device, a transmitter is provided with a dual axis acceleration sensor (hereinafter referred to as G sensor) that detects accelerations along two axes, such as an acceleration in a direction of rotation of a wheel and an acceleration in a radial direction of the wheel. Because a phase of a detection signal of each axis shifts according to the direction of rotation of the wheel, it is determined whether the transmitter is associated with a left wheel or a right wheel by comparing the phase difference of the detection signals.